At the molecular level, taste is one of the least understood sensory perceptions. A taste sensation is initiated by the interaction of a taste compound with taste receptors of taste buds on the tongue. One of the most studied tastes is sweetness. Sweet taste can be elicited to varying degrees by a wide variety of compounds such as mono- and di-saccharides, several amino acids, dipeptide derivatives such as aspartame, glycerol, cyclamates, saccharine, and some hydrated inorganic compounds. Comparison of the structures of these sweet compounds revealed no clues about what may be the chemical determinants of sweet taste. Furthermore, due to the low specificity of these compounds for the sweet receptors, it has been difficult to study the sweet taste sensory perception at the molecular level. Recent discovery of two unusual proteins, monellin and thaumatin, newly inspired the search for sweet determinants. These proteins are a few hundred thousand times sweeter than sugar on a molar basis and three to five thousand times sweeter on a weight basis. Such a high specificity makes these molecules the best system to study the interaction between the sweet receptor and its ligand. Our overall objective is to determine the 3-dimensional structures of monellin and thaumatin by x-ray crystallographic methods to lay a structural foundation for future studies on taste sensory perception at the molecular level.